The invention relates to a device for measuring an electric current by Faraday effect comprising:
a light source supplying an incident light beam,
an input polarizer to polarize the incident light beam linearly,
a magneto-optical transducer receiving the polarized incident light beam,
a beam separator arranged at the output of the transducer and supplying first and second output light beams,
an output polarizer arranged on the path of the first output light beam,
a processing unit comprising a first optical input connected to the output of the output polarizer, a second optical input receiving the second output light beam directly from the beam separator, and means for opto-electronic conversion connected to the first and second optical inputs to supply first and second electrical signals to computing means for digital computation of the current to be measured.
In a Faraday effect current sensor, the polarization plane of a polarized incident light undergoes a rotation which is a function of the magnetic field created by the electric current to be measured. The current to be measured can be determined by determining the angle of rotation of the polarization plane of the light on output of the optical sensor.
In the absence of specific measurements, measurement is sensitive to the optical power drifts of the sensor, to temperature variations, to vibrations of the sensor and to the optical and electronic noise in the whole of the measuring chain.
In most known Faraday effect current sensors, a polarization analyzer breaks the optical beam output from the magneto-optical transducer down into two optical components polarized linearly along orthogonal axes. These optical components are converted into electrical signals, the current to be measured being able to be determined from analysis of these signals.
The document FR-A-2,686,422 describes a sensor of this type wherein the electrical signals representative of the two optical components are processed by two distinct measuring channels before being applied to the computing means. In one of the measuring channels the electrical signals are applied to a variable gain amplifier. The sensor then computes the ratio xcex94/xcexa3 between the difference xcex94=I1xe2x88x92GI2 and the sum xcexa3=I1+GI2, wherein I1 and I2 are respectively representative of the intensities of the two optical components and G is the gain of the variable gain amplifier. This sensor thus enables optical drifts and the optical noise up-line from the polarization analyzer to be eliminated.
The document WO-A-9510046 describes a sensor wherein the electrical signals S1 and S2 representative of the two optical components are standardized to form a signal P=(S1xe2x88x92S2)/(S1+S2). The AC component PAC and DC component PDC of this signal are used to calculate a signal compensated in temperature, preferably in the form PAC/(1+KPDC), where K is a correction coefficient.
Furthermore, the article xe2x80x9cA common-mode optical noise-rejection scheme for an extrinsic Faraday current sensorxe2x80x9d, by Fisher et al. (1996, IOP Publishing Ltd), describes an optical current sensor eliminating the common optical noise induced by the vibrations of connecting optical fibers. The optical beam output from the magneto-optical transducer is divided into two beams by a separator adjoined to the transducer. The two beams, one thereof passes through a polarizer, are guided by optical fibers to two photodiodes. The current to be measured is calculated from the difference between the electrical signals supplied by the photodiodes. This type of sensor does not enable either the differential optical noise, or the drifts, or the noise due to the electronic processing circuit to be compensated, nor does it compensate the temperature variations.
The optical sensor described in U.S. Pat. No. 5,008,611 is rendered insensitive to birefringence variations, due in particular to temperature variations, by a suitable choice of the angle made by the direction of polarization of the input polarizer with a principal axis of the magneto-optical transducer. In this document, the linearly polarized optical beam output from the transducer is not separated into two components. The current to be measured is computed from the ratio between the AC and DC components of an electrical signal representative of the intensity of the polarized output optical beam. This type of sensor remains sensitive to optical noise in the measurement passband.
The object of the invention is to achieve a device for measuring an electric current by Faraday effect not presenting the shortcomings of known devices.
This object is achieved by the fact that the computing means comprise means for computing a first quantity constituted by the ratio between the first and second electrical signals, means for determining the AC and DC components of the first quantity, means for computing a second quantity from the AC and DC components of the first quantity, and means for computing the current to be measured from the second quantity.
According to a first development of the invention the second quantity is obtained by computing the ratio between the AC and DC components of the first quantity.
According to a second development of the invention the second quantity S is obtained according to the equation:   S  =            R      AC              1      +              a        ⁡                  (                                    R              DC                        -            1                    )                    
wherein RAC and RDC are respectively the AC and DC components of the first quantity and a is an adjustment coefficient.
The beam separator is preferably located as close as possible to the processing unit and is connected to the transducer by a single-mode, polarization maintaining optical fiber. The influence of differential optical noises can thus be minimized.
According to another development of the invention the means for opto-electronic conversion comprise two photodiodes formed on a single semi-conductorg substrate and associated respectively to the first and second optical inputs of the processing unit, two amplifiers formed on a single semi-conductor substrate being connected on output of the photo-diodes. The differential electronic noises and drifts are thus minimized.
According to another development of the invention, the means for opto-electronic conversion comprise first and second photodiodes connected to the first and second optical inputs of the processing unit, the processing unit comprising first and second amplifiers respectively connected to the outputs of the first and second photodiodes, means for determining the DC component of the output signals of one of said amplifiers, means for determining a first difference between the output signals of the first amplifier and of the means for determining the DC component of the output signals of one of said amplifiers, means for determining a second difference between the output signals of the second amplifier and of the means for determining the DC component of the output signals of one of said amplifiers, the outputs of the means for determining the DC component of the output signals of one of said amplifiers and of the means for determining the first and second differences being connected to inputs of an electronic digital processing circuit comprising means for reconstituting, from the signals applied to its inputs, the first and second electrical signals used by the means for computing the first quantity, the processing unit comprising amplifiers of preset gain g, connected between the means for determining the first and second differences and the associated inputs of the electronic digital processing circuit, the first and second electrical signals Ui used by the means for computing the first quantity being determined according to the equation:
U =(A/g)+A3
with i=1, 2 and wherein A1, A2 and A3 are respectively the signals applied to the inputs of the electronic digital processing circuit.
According to another development of the invention, the means for opto-electronic conversion comprise first and second photodiodes connected to the first and second optical inputs of the processing unit, the processing unit comprising first and second amplifiers respectively connected to the outputs of the first and second photodiodes, means for determining the DC components of the output signals of said amplifiers, means for determining the AC components of the output signals of said amplifiers, the outputs of the means for determining the DC and AC components of the output signals of said amplifiers being connected to inputs of an electronic digital processing circuit comprising means for reconstituting, from the signals applied to its inputs, electrical signals used by the means for computing the first quantity, the processing unit comprising amplifiers of preset gain g, connected between the means for determining the AC components and the associated inputs of the electronic digital processing circuit, the electrical signals V1 and V2 used by the means for computing the first quantity being determined according to the equations:
xe2x80x83V1=B1+B2
V2=B3+B4
wherein B1, B2, B3 and B4 are respectively the signals applied to the inputs of the electronic digital processing circuit.